1. Field of the invention
This application is directed to the field of disc drive data storage devices and more particularly, but not by way of limitation, to a method for compensating for variations in the torque constant of a voice coil driven actuator for moving the read/write heads in a hard disc drive data storage device.
2. Brief Description of the Prior Art
Disc drives of the type referred to as "Winchester" disc drives are well known in the industry. Such disc drives incorporate a "stack" of one or more disc-shaped platters mounted on a spindle motor for constant high speed rotation. The surface of these discs is coated with a magnetizable medium for the recording of digital data in a plurality of circular, concentric data tracks.
A number of read/write heads act in cooperation with the disc surfaces for the recording and retrieval of data. These heads are attached to some sort of actuator mechanism which operates under the control of electronic circuitry to controllably move the heads from track to track.
The actuator that moves the read/write heads has taken many different forms over the years. Early Winchester disc drives for personal computers used a stepper motor to move the read/write heads in an "open loop" control scheme. This open loop control relied on the magnetic detent inherent in stepper motor construction to define the location of each data track on the disc surface. That is, once a starting track location, commonly referred to as "track zero", was located (typically closely adjacent to the outer diameter of the discs), all other tracks were located by keeping a record of the "current track" number and applying a calculated number of step pulses to the stepper motor to move the read/write heads inward or outward to the desired track location. In such an open loop control scheme, there was no "feedback" from the disc surface to ensure the repeatable accuracy of the relative location of the heads to the data tracks, thus limiting the density of the spacing between tracks to the accuracy of the stepper motor itself, which in turn was limited by the precision of the machining of the internal components of the motor. The speed of such an actuator, while improved with manufacturing and control techniques, was also quite limited.
Over the years, the market has demanded disc drives of greater capacity and faster access capability than could be achieved using stepper motors to drive the actuator. This lead to the increasing prevalence of the use of voice coil motors to drive the actuator. Early linear voice coil motors, which drove the read/write heads on a straight radial line across the disc surface, have currently been largely superceded by rotary voice coil actuators, because of their compact size and reduced moving mass, thus permitting smaller disc drive packages with faster access speeds.
A typical rotary voice coil actuator, also sometimes referred to as a voice coil motor or VCM, consists of an arrangement of permanent magnets fixed relative to the housing of the disc drive, and a coil (or coils) mounted on the movable portion of the actuator within the magnetic field of the permanent magnets. When controlled DC current is applied to the coil, a magnetic field is generated surrounding the coil which interacts with the magnetic field of the permanent magnets to force movement of the coil and actuator body on which the coil is mounted. The amount of force generated by this magnetic interaction (and thus the torque capability of the motor) is dependent on many factors, including the strength of the permanent magnets; the size and number of turns in the coil; the amount of current applied to the coil; and the proximity of the coil to the magnets. Advances in materials science, manufacturing technology and electronic controls have lead to the current generation of disc drives which use rotary voice coil actuators to provide capacities of several hundred megabytes with average access times of less than fifteen milliseconds, all in the five and one-quarter inch or smaller form factor disc.
A voice coil motor does not have inherent magnetic detent, so another technique must be used to control the movement of the read/write heads from one data track to another. This usually involves the use of a "closed loop servo" system. In a closed loop servo system, head position information is recorded on either a dedicated servo surface of one of the discs, or on specific dedicated portions of the disc surfaces used to store user data. In the case of a dedicated servo surface, a servo head constantly reads prerecorded positional information and sends this information to the electronic circuitry used to supply drive current to the motor coil as "feedback". The VCM control circuitry uses this feedback to stay centered on, or follow, a desired track, and to control "seeks" from one track to another. For disc drives using an "embedded servo", prerecorded positional information is placed on the same disc surfaces used to record user data and is read back to the actuator motor control logic only during carefully timed "windows", during which time all data reading and writing capability is disabled. Hybrid systems using both a dedicated servo surface and embedded servo information have also been used, although the penalty in lost data recording space has made this type of system rare.
One of the primary goals of all high volume manufacturers of disc drives is to produce products that are highly consistent from unit to unit without imposing such strict component control that the cost of the unit becomes prohibitive in the marketplace. Similarly, these manufacturers must make disc drives which perform uniformly over a wide range of ambient temperatures, e.g., from about 5.degree.-50.degree. C. (41.degree.-122.degree. F.), with internal temperatures exceeding these ambient temperatures by up to 20.degree. C.. These demands and goals lead to one of the more difficult challenges facing disc drive manufacturers.
A high volume disc drive manufacturer can expect to build several hundred thousand, or even several million, of the same disc drives over the life of the product. It is therefore impossible, without economically prohibitive controls, to produce perfectly uniform magnets for use in these products, and magnet strength, of necessity, can therefore be expected to vary from unit to unit by as much as +10%. Similarly, the magnetic strength within a given magnet is not absolutely uniform and will therefore cause the strength of magnetic interaction between the permanent magnets and the magnetic field of the moving coil of a voice coil actuator to vary dependent upon the relative position of the coil to the magnets. Also, the strength of the magnets can be expected to decrease over the operational life of the disc drive and to vary across the specified operational temperature range as well. Furthermore, tolerance variations allowed in the electronic components supplying the drive current to the coil in a voice coil motor create yet another variable which impacts the motive force of the motor. Other mechanical variables in the actuator mechanism of the disc drive, such as the precision of the ball bearings used to guide the motion of the actuator, make it extremely difficult to produce large numbers of very uniform disc drives.
It is therefore desirable that a way be found to compensate for normal expected variations in the magnetic and electronic components in a voice coil actuator for a disc drive.